@Minutoh
It's pretty rare to see her depicted with them (she doesn't have them in PCB, then does have them in IaMP, then doesn't in IN). I think it was used here to define her simply by her hand alone.
Mima never shows up again because of Yukari, now everything makes sense.
@Minutoh
Well, probably because /y/ is already the yaoi board... another possible(?) explanation would be the fact that people usually use just "u" as short for "you", plus, it kinda sound like "yu", resulting in something like "u-ri" for "yuri"... but well, it's just a guess.